In a medium saucepan, add the cherries and the sugar. Using a muddler or the back of a wooden spoon, muddle the cherries into the sugar until the cherries' juices have been extracted. Add the vinegar and let the mixture sit for an hour to macerate.
Add ½ cup water to the pan and place over medium heat. Bring to a boil, stirring occasionally. Remove the pan from heat. Strain the syrup into a lidded container and store in the refrigerator until using.